The production of vesiculated polymer granules and their utility as pigments or paints and other compositions has been described in the literature, e.g., see Kershaw Australian OCCA Proceedings and News, August 1971, pages 4-6, 8, 9, Kershaw et al. Canadian Pat. No. 856,861 issued Nov. 24, 1970, and Fox U.S. Pat. No. 3,740,255. As disclosed therein the vesiculated polymer granules, preferably spheroidal granules, have a cell-like structure, the walls of which are provided by the polymer. The granules contain a plurality of cells or vesicles; that is, they are not mono-cellular or balloon-like. Ideally each vesicle is encased in a complete shell of polymer, i.e., the polymer granule does not have a continuous porosity extending from one cell to another, but consists of a plurality of discrete isolated vesicles. A pigment may be provided in the polymer walls or in the vesicles. The ratio of the diameter of the granule to the mean individual vesicle should be at least 5 to 1, the vesicles should occupy from 5 to 95% of the total volume of the granules, and the mean diameter of the granules may be up to 500 microns or more but granules having a mean diameter of 1 to 100 microns are preferred for opacifying agents.
Canadian Pat. No. 877,357 (granted Aug. 3, 1971) describes the production of vesiculated polymer granules converting to solid polymer a liquid medium in which is dispersed particles of another polymer swollen by a liquid swellant, which is itself dispersed in a suspending liquid. The preformed emulsion is polymerized by externally heating the emulsion to temperatures as high as 90.degree.C. in a sealed container for several hours.
Canadian Pat. No. 884,358 (granted Oct. 26, 1971) describes the production of vesiculated cross-linked polyester resin granules by suspending a solution of a carboxylated unsaturated polyester resin in a water-insoluble monomer in an aqueous phase in the presence of a strong base and subsequently polymerizing the resin by externally heating the suspension to about 95.degree. C. for a few hours. Prior to polymerization, the emulsion can be formed in one step by mixing the polyester resin solution with the aqueous phase or in two steps by predispersing an aqueous liquid within the polyester resin solution afterwhich the predispersed solution is mixed with a continuous aqueous phase.
Von Bonin et al. U.S. Pat. No. 3,255,127 describes the production of vesiculated thermoplastic polymers by first forming a water-in-oil emulsion, then dispersing the emulsion in an aqueous phase in the presence of a polymerization catalyst. The oil phase is polymerized by slowly agitating the dispersion at temperatures from 10.degree. to 95.degree.C. for periods 1 to several days.
While the techniques given in the above-discussed art are useful, the resultant vesiculated polymer granules tend to have certain deficiencies, at least for applications where a high quality opacifying pigment is desired. In the case of emulsions contained in an externally heated vessel or even vessels having some form of immersion heater, the oil phase globules in those portions of the emulsion nearest the heat source apparently become most susceptible to coalescence or agglomeration with the result that particle size tends to be unduly large and particle size distribution unduly wide. In fact, the heat source often tends to become coated with agglomerated particles. Where polymerization is effected slowly, with or without heating, the oil phase droplets have a tendency to agglomerate, even when the dispersed oil phase droplets are carefully agitated, resulting in a nongranular or blocky product. Since totally enclosed pores are desirable for maximum hiding power is vesiculated polymers, it will be understood that manual or mechanical breaking, grinding and other deagglomeration procedures commonly practiced in the pigments industry are not altogether satisfactory for use with vesiculated polymers as such procedures contribute to the breakdown of the enclosed pores.